In recent years, with the increase in performance of electric/electronic components (higher efficiency and more compact size), and also for bodies formed from insulated soft magnetic metal powder used for motor cores, toroidal cores, and the like, it has been demanded that iron loss be decreased, and the magnetic permeability be increased. In order to enhance the magnetic permeability, a reduction in the thickness of the insulation layer to narrow the spacing between particles of soft magnetic metal powder is required. Iron loss is generally made up of hysteresis loss and eddy-current loss, and hysteresis loss varies depending upon the type of soft magnetic material, the concentration of the impurities, work stress, and the like. The eddy-current loss varies depending upon the specific resistance for the soft magnetic material, and the degree of integrity of the insulating film. From such viewpoints, the following techniques for obtaining bodies formed from insulated soft magnetic metal powder have been proposed.
The patent literature 1 discloses a method for manufacturing soft magnetic members by a powder metallurgy technique. The iron particles are wrapped with an insulating phosphate layer, and then compressed, which is followed by applying a heat treatment to them at a heat treatment temperature with an upper limit of 600 deg C., in an oxidizing atmosphere.
In the patent literature 2, a method for compression molding iron powder and applying a heat treatment thereto in order to obtain magnetic core members having improved soft magnetism is disclosed. The iron powder is made up of fine particles which are insulated by a thin layer of low phosphor content. According to the patent literature 2, the compression molded iron powder is subjected to a heat treatment at a temperature of 350 to 550 deg C. in an oxidizing atmosphere. According to the same invention, the heat treatment should be carried out at a temperature of 350 to 550 deg C., preferably at 400 to 530 deg C., and the most preferably at 430 to 520 deg C., however, the invention as disclosed in the patent literature 2 does not surpass the invention according to the patent literature 1.
The invention according to patent literature 3 specifies that, in order to obtain a compacted core of a ferromagnetic metal powder that has reduced eddy-current loss and has mechanical strength, phosphoric acid be deposited on the surface of the ferromagnetic metal particles, and the ferromagnetic metal powder be subjected to pressurized forming, and heat treatment at 300 to 600 deg C., preferably at 400 to 500 deg C.
The invention according to patent literature 4 provides a method for manufacturing a composite magnetic material obtained by compression molding a mixture made up of a magnetic powder and an insulation material, and then carrying out heat treatment, wherein the heat treatment is carried out two or more times, and if the oxygen concentration in the atmosphere for the first heat treatment is designated P1, and the oxygen concentration in the atmosphere for the second heat treatment is designated P2, by meeting the relationship P1>P2, a composite magnetic material which is low in core loss and high in magnetic permeability, and has an excellent DC bias characteristic is obtained. If the first heat treatment temperature is designated T1 and the second heat treatment temperature is designated T2, the relationship of T1<T2 should be met, and for oxygen concentration, the relationships, 1%<_P1<—30%, and P2<—1% should be met. For heat treatment temperature, the relationships, 150 deg C.<_T1<—500 deg C., and 500 deg C.<_T2<—900 deg C. should be met. In the first heat treatment, an oxidation insulating film is formed, and in the second high temperature heat treatment, stress be relieved. However, at the time of the second high temperature heat treatment, there is a possibility that the difference in thermal expansion coefficient between the magnetic powder and the oxidation insulating film may destroy the insulating film.
The invention according to the patent literature 5 provides a coated iron-based powder with which the surface of the iron-base powder particles is coated with a coating material, wherein the amount of the coating material for the coated iron-base powder is 0.02 to 10% by mass, and the coating material is made up of glass of 20 to 90% by mass, and a binder of 10 to 70% by mass, or alternatively insulating and heat-resistant substances, other than the glass and binder, of 70% or less. The binder is preferably made up of one type or two or more types selected from silicone resin, a metal phosphate compound, and a silicate compound. No claims directed towards heat treatment are given, but in the examples, a nitrogen gas atmosphere is used at a maximum temperature of 700 deg C.
The invention according to the patent literature 6 provides a composite magnetic material comprising a plurality of composite magnetic particles having metal magnetic particles and an insulation film surrounding the surface of the metal magnetic particles, wherein the plurality of composite magnetic particles are bound to one another, and the metal magnetic particles are made up only of a metal magnetic material, and impurities in proportion of the metal magnetic particles of 120 ppm or lower. It is specified that the composite magnetic material obtained by pressure molding be subjected to stabilization heat treatment at a temperature of from 200 deg C. to the thermal decomposition temperature for the resin added, in an oxidizing atmosphere or an inert gas atmosphere.
Patent literature 1: Germany Patent No. 3439397    Patent literature 2: Japanese National-Phase Publication No. 9-512388/1997    Patent literature 3: Japanese Patent Laid-Open Publication No. 7-245209/1995    Patent literature 4: Japanese Patent Laid-Open Publication No. 2000-232014    Patent literature 5: Japanese Patent Laid-Open Publication No. 2004-143554    Patent literature 6: Japanese Patent Laid-Open Publication No. 2005-15914